Several new features are added for the long term HSPA (high speed packet access) evolution in order to meet the requirements set by the IMT-A (International Mobile Telecommunications Advanced). The main objective of these new features is to increase the average spectral efficiency.
One possible technique for improving downlink spectral efficiency would be to introduce support for multiple antennas employed at the transmitter and the receiver. For example, a four branch MIMO (multiple input multiple output) can utilize up to four Tx (transmit) and Rx (receive) antennas to enhance the SM (spatial multiplexing) gains and to offer improved beam forming capabilities. Four branch MIMO provides up to 84 Mbps per 5 MHz carrier for high SNR (signal-to-noise ratio) users and improves coverage for low SNR users.
The current HSDPA system (Release 7-10) supports one or two transmit antennas at the Node B. In these systems, from channel sounding, the UE (user equipment) measures the channel and reports the CSI (channel state information) in one sub frame to the Node B as feedback. Typically, the CSI report includes a CQI (channel quality indicator) which explicitly indicates a preferred rank and a PCI (precoding control index). The UE sends this report periodically for every subframe or TTI (transmit time interval) as feedback. Once the Node B receives this report, it grants modulation and coding, number of codes, rank and the precoding channel indicator to each specific UE based on a scheduler metric.
Currently, a 4-Tx transmission scheme for HSDPA is being discussed within the 3GPP (3rd Generation Partnership Project) standardization process. One issue the 3GPP has discussed extensively is on the design of the precoding codebook. It was decided to use codebook with 16 elements for each rank.